Torque sensors for detecting torque exerted about a predetermined rotation axis have been widely used for a variety of transport machinery and industrial machinery. For example, Japanese Unexamined Patent Publication No. 2009-058388 discloses a torque sensor of a type in which mechanical deformation caused by exertion of torque is detected by a strain gauge. Moreover, Japanese Unexamined Patent Publication No. 2007-024641 discloses a sensor for detecting torque exerted on a shaft by forming a magneto-strictive film through plating on the shaft surface and measuring a change in the magnetic properties of the magneto-strictive film. On the other hand, Japanese Unexamined Patent Publication No. 2009-244134 discloses a torque sensor of a type in which a magnetism generating section is provided at an end portion of a torsion bar, and a change in the magnetic flux density of magnetism generated by the magnetism generating section is detected by use of a magnetic flux collecting ring, and Japanese Unexamined Patent Publication No. 2006-292423 discloses a torque sensor of a type in which a large number of magnets are disposed in a cylindrical shape so that the N poles and S poles are lined up alternately in the circumferential direction, and magnetic fields generated by these magnets are detected. Further, Japanese Unexamined Patent Publication No. 2000-019035 discloses a torque sensor for which a link mechanism for deforming in the radial direction the shape of an annular member by exertion of torque is prepared, and a force applied in the radial direction of the annular member is detected based on deformation thereof by a load sensor.
In the industry, there is a demand for a small-sized torque sensor having high rigidity and a simple structure. In particular, in industrial equipment for performing automatic assembly by use of a robot arm, it is essential to monitor a force generated at a tip portion of the arm and control the same. For such a torque feedback-type robot arm, a small-sized torque sensor having high rigidity and a simple structure suitable for use integrated into a joint part of the arm is desired.
Generally, because the joint part of a robot arm has a short axial length, it is preferable that a torque sensor to be integrated into the joint part also has a small size with as short an axial length as possible. However, it is difficult to design the conventional type of torque sensor using a torsion bar with a short axial length. This is because, when torsion is produced in the torsion bar by exertion of torque, a change in axial length is caused by the torsion, and thus it is inevitable to set the overall length of the torsion bar so long that the change can be disregarded.
Usually, when the axial length of a torque sensor integrated into a joint part changes due to exertion of torque, a force in an axial direction is newly produced to induce eccentricity or an inclination of the axis and serve as a factor to hamper accurate torque detection. For coping with such a harmful effect, a unique mechanism for absorbing a change in axial length is required. But, provision of such a mechanism complicates the structure accordingly, which is not preferable. Ultimately, it is inevitable to set the overall length of the torsion bar so long that a change in axial length can be disregarded, so that it is difficult to design the conventional torque sensor with a short axial length.
Moreover, for performing stable torque feedback, it is necessary to secure high-speed response of signal processing from an electrical point of view as well as to secure high rigidity in a sensor structure from a mechanical point of view. There is a basic principle of a common torque sensor in which mechanical torsion is produced in some structure and the torsion is detected electrically. Therefore, as an object in which torsion is produced, of course, a perfectly rigid body cannot be used, and it is necessary to use a structure that produces some extent of elastic deformation. However, if a torsion angle produced as a result of torque detection is large, frequency characteristics of a control system deteriorate due to the hunting phenomenon, and high-speed response can no longer be secured.
Therefore, for performing stable torque feedback, it is necessary to minimize the torsion angle to be produced as a result of torque detection, and it is preferable to construct the torque sensor by a structure having as high a rigidity as possible. However, with the conventionally proposed magnetic methods for detecting torsion angles, it is difficult to detect a small torsion angle at high accuracy.
Further, from the point of view of industrial application, a cost reduction is of course required also for torque sensors. For that purpose, it is desirable to simplify the structure as much as possible. However, in the conventionally proposed torque sensors, it is necessary to adhere a strain gauge to a torsion bar, form a magneto-strictive film, or attach magnets or coils, so that the structure is inevitably considerably complicated.
It is therefore an object of the present invention to provide a small-sized torque sensor having high rigidity and a simple structure.